1. Field of the Invention
The present invention relates to a heat-sensitive recording material, and more particularly, to a heat-sensitive recording material which has excellent transparency, does not cause sticking or generation of noise during recording, provides high quality images suitable for a recording medium used in the medical field and exhibits a broad dynamic range.
2. Description of the Related Art
Heat-sensitive recording has been widely used recently in various fields because the heat-sensitive recording has advantages such as: (1) developing processing is not necessary; (2) recording materials having a quality close to that of general use paper are obtained when paper is used as the substrate; (3) handling is easy; (4) a high developed color density of coloring can be obtained; (5) simple, reliable and inexpensive recording apparatuses can be used; (6) noise is not generated during recording; and (7) no particular maintenance is required. Heat-sensitive recording is increasingly used, for example, in the field of facsimiles and printers and in the field of labels such as POS.
As the heat-sensitive material used for the above heat-sensitive recording, heat-sensitive recording materials utilizing the reaction of colorless electron-donating dyes and electron-accepting compounds and heat-sensitive recording materials utilizing the reaction of diazo compounds and couplers have been widely known.
Under these circumstances, a transparent heat-sensitive recording material which can be recorded directly by a thermal head is desired recently so that multicolor images can be recorded and recorded images can be projected by an overhead projector or can be directly observed on a light table.
It is proposed that a heat-sensitive recording material be prepared by disposing, on a transparent substrate such as a film of a synthetic polymer substance, a heat-sensitive recording layer which is formed by dispersing a substantially colorless color forming component A and a substantially colorless color forming component B which develops color by reaction with the color forming component A in a binder as fine particles or by using either one of components A and B which are micro-encapsuled and the other component in the form of an emulsion.
Although the above heat-sensitive recording material has excellent transparency, the above heat-sensitive recording material has a drawback in that sticking occurs and noise is generated when the heat-sensitive recording material is used for forming images using a heat-sensitive recording apparatus such as a thermal recording printer. To overcome the drawback, it is proposed that a protective layer composed of a pigment and a binder as main components is disposed on the heat-sensitive recording layer of the heat-sensitive recording material.
However, when the above transparent heat-sensitive recording material is used as an image outputting medium in which a particularly high density of black color is required, the heat-sensitive recording material has a problem in that a thermal head becomes extremely worn. Therefore, circuits in the thermal head are broken or recorded images have defects such as missing portions and blurrings.
In particular, when the above transparent heat-sensitive recording material is used as a recording medium in the medical field, formation of defects such as uneven distribution of density and missing portions of images during printing of images must be prevented as much as possible because a particularly high density of black color is required in images used in the medical field and delicate differences in the density of images are detected as signals and used for diagnosis. In general, when a thermal head is used for recording, the thermal head is designed so as to have a large dynamic range, i.e., a large energy range required for obtaining the saturated transmission density D.sub.T-max, to reduce fluctuations in the density caused by slight differences in the thermal conductivity between heating resistors in the head. Therefore, the energy is applied to the thermal head for a longer time during printing. Moreover, a high density of black color is required as described above. Thus, the thermal energy applied during printing is markedly higher than that in general use facsimiles and label printers and the thermal head is used under a very disadvantageous condition from the standpoint of wear of the thermal head.
To reduce wear of a thermal head, it is generally attempted to add a small amount of coarse grains (hereinafter referred to on occasion as a matting agent) to a protective layer of a heat-sensitive recording material to make the area of contact between the thermal head and the heat-sensitive recording material smaller or to add various types of lubricants to decrease the friction coefficient between the thermal head and the heat-sensitive recording material.
However, the addition of a matting agent adversely affects tight contact between the thermal head and the heat-sensitive recording material to cause uneven heat transfer during printing. Therefore, reproducibility of dots in the color developed area decreases to form uneven density in images and it not preferable that this method is used for a recording medium for producing high quality images such as a medical recording medium. When a lubricant is added, the added lubricant adheres to the thermal head, resulting in damage and uneven densities of the recording medium. Therefore, the addition of a lubricant is not preferable.